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1. Field of the Invention
The invention pertains to the general field of printing and in particular to the speed, reliability and reproduction quality of inkjet printing.
2. Background of the Invention
Ink jet is a low cost and effective method for deposition of any material in fluid form in numerous applications, mainly in printing. It has made the entire revolution in desk-top publishing possible and has become the mainstay color printing technology for home office use.
Ink jet printing, however, suffers form a number of drawbacks. The printing speeds achievable do not in general match those achievable using traditional offset printing, nor does inkjet printing match offset printing as regards printing quality attainable.
As regards print quality, inkjet printing is often characterized by a distinctive banding pattern that is repeated over the printed image. This may be traced to the very arrangement of the inkjet nozzles in the printing head. Relatively small nozzle misalignments or off-center emission of droplets are often at the root of this problem. As the printing head is translated laterally across the width of the printing surface, the visual imperfections are therefore repeated with perfect periodicity, producing the characteristic inkjet printer banding or striping. A number of approaches exist to address this matter, but they invariably have a negative effect on the throughput of the printer as a whole. This is a debilitating price to pay in the volume printing industry where time and throughput are of the essence. There is a clear need for a method that addresses visual imperfections in inkjet printing, of which banding is just one example, without compromising throughput.
A further point in the arena of print quality is the matter of xe2x80x9cwickingxe2x80x9d or xe2x80x9crunningxe2x80x9d. The water-based ink typically employed in ink-jet printers tends to xe2x80x9crunxe2x80x9d along the fibers of certain grades of paper. This phenomenon is also referred to as xe2x80x9cwickingxe2x80x9d and leads to reduced quality printing, particularly on the grades of paper employed in volume printing. The final printed dot is often much larger than the droplet of ink emerging from the inkjet nozzle and the integrity of the dot is lost in the process.
In order to obtain better quality prints from inkjet printers it is therefore often necessary to employ specially treated paper at high unit cost in order to ensure that the ink deposition process is under greater control during printing. This issue is directly traceable to the low viscosity of water-based inks. There is a clear need to be able to print on papers having a wider range of paper quality using low viscosity inkjet inks.
The linear printing speed of inkjet printing is of the order of 10 times slower than offset printing and in an industry where throughput and time are dominant considerations. This represents a major issue limiting the implementation of inkjet technology in industrial printing systems. The inkjet printing speed limit is dictated by the rate at which the miniature inkjet ejection capsules can eject ink in discrete controllable amounts. This rate is at present of the order of 20,000 pulses per second. This limits state of the art inkjet printers to print rates of the order of 2 pages per second, falling far short of the offset printing rate. For inkjet printing to be implemented on a wider scale in industry, the printing throughput must therefore be increased.
The matter of failure in nozzles is also deserving of attention. Many approaches exist for detecting faulty inkjet nozzles and for re-addressing the inkjet printing head in order for other nozzles to perform the task of the faulty one. This includes various redundancy schemes. Again, these usually have the effect of slowing down the net printing process speed. In many cases the redundancy is managed at printing head level, requiring backups for entire printing heads. This adds to the cost of the technology per printed page and again limits the industrial implementation of the technology. There is a clear need for the backup nozzles at lower cost per printed page and without reducing the throughput.
The prior art describes various array inkjet print head designs aimed at reducing inkjet-printing artifacts such as banding. Examples are Furukawa in U.S. Pat. No. 4,272,771, Tsao in U.S. Pat. No. 4,232,771, Padalino in U.S. Pat. No. 4,809,016 and Lahut in U.S. Pat. No. 5,070,345. Considerable work has also been done in addressing reliability by providing inkjet nozzle redundancy. Examples are Schantz in U.S. Pat. No. 5,124,720, Hirosawa in U.S. Pat. No. 5,398,053 and Silverbrook in U.S. Pat. No. 5,796,418. Transfer rollers have also been described, both with and without the droplets deposited on them being processed in some way before final printing in order to reduce wicking. See for example Takita in U.S. Pat. No. 4,293,866, Durkee in U.S. Pat. No. 4,538,156, Anderson in U.S. Pat. No. 5,099,256, Sansone in U.S. Pat. No. 4,673,303 and Salomon in U.S. Pat. No. 5,953,034.
This invention provides methods for printing inkjet patterns with high dot integrity on a wide range of media. The methods comprise depositing fluid droplets which nay comprise ink droplets from fluid droplet sources onto an intermediate transfer surface. The methods change the properties of the ink droplets after they have been emitted from the fluid droplet sources. Changing the properties of the droplets may comprise decreasing their size and increasing their viscosity. Dots immediately adjacent to one another in the pattern may be printed in separate passes to retain dot integrity.